Different complexation Behavior of Fe(III), Co(II) and Ni(II) with pyridine-2,6-dicarboxylic acid and 4,4′-Bipyridine adduct: Syntheses, crystal structures and solution studies

Three complexes (bpyH)[Fe(pydc) 2 ].4H 2 O ( 1 ), [Co 2 (pydc) 2 (μ-bpy)(H 2 O) 4 ].4H 2 O ( 2 ) and [Ni(pydc)(bpy)(μ-bpy)] 4 .8H 2 O ( 3 ) were prepared from the reaction between pyridine-2,6-dicarboxylic acid, pydcH 2 , and 4,4′-bipyridine, bpy, with FeCl 2 .4H 2 O, CoCl 2 .6H 2 O and NiSO 4 .6H 2 O, respectively. The complex ( 1 ) crystallizes in the space group of the triclinic system and contains two molecules per unit cell. The structure has been refined to a final value for R factor of 0.0355 based on 4922 reflections. The crystal structure shows that Fe(III) is coordinated to two pyridine-2,6-dicarboxylate ligands. The complexes ( 2 ) and ( 3 ) crystallize in the space group P 2 1 / c of the monoclinic system and contain two molecules per unit cell. The structure ( 2 ) has been refined to a final value for R factor of 0.0295 based on 2249 reflections. The molecular structure ( 2 ), which is a binuclear complex, contains two pyridine-2,6-dicarboxylate ligands, and a 4,4′-bipyridine bridge ligand. The structure ( 3 ) has been refined to a final value for R factor of 0.0579 based on 7721 reflections. In this structure, which is a tetramer complex, pyridine-2,6-dicarboxylate and 4,4′-bipyridine fragments act as a tri- and bidentate ligands, respectively. The protonation constants of 4,4′-bipyridine and pyridine-2,6-dicarboxylate, the equilibrium constants for the pydc-bpy system and the stoichiometry, and stability of complexation of this system with Fe 3+ , Co 2+ and Ni 2+ ions in a aqueous solution were investigated by potentiometric p H titration method. The stoichiometry of the most complexes species in solution was found to be very similar to the crystalline cited metal ion complexes.